1. The Field of the Invention
The present invention generally relates to x-ray generating devices. In particular, the present invention relates to an apparatus for preventing non-uniform attenuation of an x-ray beam by bubbles formed in the cooling fluid of an x-ray generating device.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, and then impacted upon a material of a particular composition. This process typically takes place within an evacuated enclosure of an x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a reservoir for a fluid, such as dielectric oil, that serves both to cool the x-ray tube and to provide electrical isolation between the tube and the outer housing.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted as a beam through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam is then directed toward an x-ray subject, such as a medical patient, so as to produce an x-ray image.
Generally, only a small portion of the energy carried by the electrons striking the target surface of the anode is converted to x-rays. The majority of the energy is rather released as heat. To help dissipate this heat, the cooling fluid disposed in the outer housing assists in absorbing heat from surfaces of the x-ray tube and removing it from the x-ray device. This heat removal can be accomplished, for example, via radiation of the heat from the outer surface of the housing, or by continuously circulating the cooling fluid through a heat exchanger.
Despite the overall success of the cooling fluid in dissipating heat from the x-ray tube, however, certain areas within the x-ray device may not be adequately cooled. One of these areas is located between the respective windows of the x-ray tube and outer housing. Because of this, extreme heating of the cooling fluid in this localized region may occur. This extreme heating can exceed the ability of the cooling fluid to remove the heat. Consequently, intermittent boiling of the cooling fluid can occur in the localized region between the two windows, creating air bubbles within the fluid that tend to congregate on the inner surface of the outer housing window.
The accumulation of bubbles at the inner surface of the outer housing window is undesirable for several reasons. Principal among these relates to the fact that the air bubbles present in the cooling fluid at the window surface possess a distinct density, and thus a distinct rate of x-ray attenuation, from the fluid itself. Because of this density difference, x-rays passing through a bubbly fluid region will be attenuated a different rate than x-rays passing through a fluid-only region. Thus, bubbles that are created by intense heating of the cooling fluid and are randomly distributed on the inner surface of the outer housing window create a non-uniform attenuation of the x-ray beam that passes through the window. The result is a non-uniform x-ray beam exiting the x-ray device, which in turn produces inferior results for the particular application for which the device is being used. For instance, in medical imaging a non-uniform x-ray beam can cause the image quality and clarity of the radiographic images produced thereby to substantially decrease. For this and other reasons, bubbles present at the inner surface of the outer housing window are highly undesirable.
Non-uniform x-ray beam attenuation can be further exacerbated by an additional factor combining with the accumulation of bubbles on the outer housing window inner surface. As mentioned, many x-ray devices are utilized in connection with medical imaging systems, such as CT scanners. In such systems, the x-ray device is typically mounted on a gantry that spins at high speeds during the scanning process. This spinning subjects the x-ray device and its components to various rotationally related forces. These dynamic rotational forces are not of such a nature as to completely displace fluid bubbles formed at the surface of a typical housing window. However, these forces are sufficient to cause bubbles at the window surface to oscillate during gantry rotation. This bubble oscillation further increases the uneven attenuation of the x-ray beam, resulting in even more non-uniform beam characteristics.
In light of the above discussion, it would be generally desirable to produce an x-ray tube having superior beam characteristics. Particularly, a need exists for an x-ray device, and, more particularly, a housing window that is designed to eliminate the collection of cooling fluid bubbles on the housing window in order to reduce non-uniformity for the x-ray beam, especially in high-rotational environments.